3 Major Classifications of Carbohydrate and its Significance

Some of the major groups under which the carbohydrates are divided with its chemical structures and functions are as follows:

The carbohydrates, or saccharides are most simply defined as polyhydroxy aldehydes or ke­tones and their derivatives.

Organic compounds in this group are so called because they consist of carbon hydrogen and oxygen, the last two in a 2:1 ratio.

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The carbohydrates are widely distributed both in animal and plant tissues. In animal cells, they occur chiefly in the form of glucose and glyco­gen, whereas in plants, cellulose and starch are their main representatives.

I. Monosaccharides (Simple Sugars):

Monosaccharides are those sugars which cannot be hydrolysed into a simpler form. They have the empirical formula (CH2O)n. The simplest monosaccharide’s are the three carbon triosesglyceraldehyde and dihydroxyacetone.

Depending upon the number of carbon atoms they possess, simple sugars may be subdivided into different classes such as trioses, tetroses, pentoses, hexoses or heptoses; and as aldoses or ketoses, based upon whether the aldehyde or ketone groups are present. Examples are:

Aldoses

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Ketoses

Trioses (C3H6O3)

Glyceraldehyde

Dihydroxyacetone

Tetroses (C4H8O4)

Erythrose

Erythrulose

Pentoses (C5H10O5)

Ribose

Ribulose

Hexoses (C6H12O6)

Glucose

Fructose

Within each category, sugars are further distinguished according to the left or right alignment of the H and OH groups around the carbon atom adjacent to the terminal primary alcohol carbon {e.g. carbon 5 in glucose). When the OH group on this carbon is on the right, the sugar is a member of the D (Dextrorotatory) series; when it is on the left it is a member of L (Laevorotatory) series.

Most of the monosaccharides occurring in mammalian metabolism are of the D configuration. Compounds which have the same structural formula but differ in atomic configuration are known as stereoisomers.

Monosaccharides of Physiologic Importance:

A. Trioses:

They are formed in the body during the metabolic breakdown of the hexoses. Ex­amples are glyceraldehyde and dihydroxyacetone.

B. Pentoses:

They are important constituents of nucleic acids, and many coenzymes. They are also formed as intermediates during certain metabolic processes. Examples are Ribose which is a structural element of ATP, nucleic acids and coenzymes NAD, NADP and flavo-proteins; Ribulose; Arabinose and Xylose.

C. Hexoses:

They are physiologically the most important of the mono-saccharides, e.g., Glucose, Fructose, Galactose and Mannose.

(i) Glucose:

It is found normally in the fruit juices and formed in the body by hydrolysis of starch, cane sugar, maltose and lactose. Glucose is the “Sugar” of the body. Structure of glucose may be depicted as a chain or ring forms (-pyranose-hexagon structure with five carbons and one oxygen; and furanose-has a pentagon structure with four carbons and one oxygen).

(ii) Fructose:

It occurs naturally in fruit juices and honey. Hydrolysis of cane sugar in the body also yields fructose.

(iii) Galactose:

A constituent of glycolipids and glycoproteins, it is synthesized in the mammary glands and hydrolyzed to make the lactose of milk.

(iv) Mannose:

It is obtained on hydrolysis of plant mannosans and gums. Mannose is a constitu­ent of prosthetic polysaccharide of albumins, globulins and mucoproteins.

Pentoses and hexoses exist in both open chains as well as ring forms.

Derived Monosaccharides:

Monosaccharides are modified variously to form a number of different substances. The important derivatives are:

(i) Deoxy Sugar:

Deoxygenation of ribose produces deoxyribose. The latter is a constituent of deoxyribotides found in DNA.

Disaccharides consist of two monosaccharides joined by a glycosidic linkage (C-O-C). Their general formula is Cn(H2O)n-1 The most common disaccharides are maltose, lactose and sucrose.

(i) Maltose:

It is formed as an intermediate product of the action of amylases on starch, and contains two D-glucose residues in 1, 4 linkage. It is found in detectable amount in most germinating seeds and tissues where starch is being broken down.

(ii) Lactose:

It is found in milk but otherwise does not occur in nature. It yields D-galactose and D-glucose on hydrolysis. Since it has a free anomeric carbon on the glucose residue, lactose is a reducing disaccharide.

(iii) Sucrose, or cane sugar is a disaccharide of glucose and fructose. The hydrolysis of sucrose to D-glucose and D-fructose is often called inversion since it is accompanied by a net change in optical rotation from dextro to levo as the equimolar mixture of glucose and fructose is formed (this mixture is often called invert sugar). This reaction is catalyzed by enzymes called invertases. Sucrose is extremely abundant in the plant world and is familiar as table sugar.

(iv) Trehalose:

It contains two D-glucose residues and is a non-reducing disaccharide as su­crose. It is the major sugar found in the haernolymph of many insects.

III. Trisaccharides:

A number of trisaccharides occur free in nature. Raffinose is found in abundance in sugar beets and many other higher plants. Melezitose is found in the sap of some coniferous trees.

Most of the carbohydrates found in nature occur as polysaccharides of high molecular weight. They are complex carbohydrates which are formed by polymerisation of large number of monosaccha­ride monomers. Polysaccharides are also called glycans.

They are long chained which may be branched or unbranched. On complete hydrolysis with acid or specific enzymes, these polysaccharides yield monosaccharides and/or simple monosaccharide derivatives. Depending upon the composition, polysaccharides are of two types: homopolysaccharides and heteropolysaccharides.

(i) Homopolysaccharides or homoglycans are those complex carbohydrates which are formed by polymerisation of only one type of monosaccharide monomers. For example, starch, glycogen and cellulose are composed of a single type of monosaccharide called glucose.

(ii) Heterpolysaccharides or heteroglycans are those complex carbohydrates which are pro­duced by condensation of either monosaccharide derivatives or more than one type of monosaccha­ride monomer, e.g., chitin, agar, peptidoglycan, arabanogalactans, arabanoxylans, etc.

Depending on their biological function polysaccharides are of three main types-storage, struc­tural and mucopolysaccharides.

A. Storage Polysaccharides:

These polysaccharides serve as reserve food. Starch, most abundant in plants and glycogen in animals are usually deposited in the form of large granules in the cytoplasm of cells.

1. Starch:

Starch (C6H10O5)x is the most important food source of carbohydrate and is found in cereals, potatoes, legumes and other vegetables. Natural starch is insoluble in water and gives a blue colour with iodine solution. It is a polyglucan homosaccharide. Starch consists of two components, amylose and amylopectin.

(a) Amylose (15-20%):

It is a non-branching, helical structure consisting of glucose residues in ∞-l, 4 linkage.

(b) Amylopectin (80-85%):

It consists of highly branched chains having 24-30 glucose residues per chain. The glucose residues are united by ∞ (1—>4) glycosidic linkage in the chain and by ∞ (1—>16) linkage at the branch points.

2. Glycogen:

It is the main storage polysaccharide of animal cells, the counterpart of starch in plant cells. Glycogen is especially abundant in the liver, where it may attain up to 10% of the wet weight. Like amyl-pectin, glycogen is a polysaccharide of D- glucose in a ∞ (1—>4) linkage. However, it is more highly branched; the branches occur about every 8 to 12 glucose residues.

The branch linkages are β (l —>6). The straight part is helically twisted with each turn having six glucose units. The distance between two branching points is 10-14 glucose residues. Glycogen is readily hydrolyzed by ∞-and β amylases to yield glucose and maltose respectively.

3. Inulin:

It is a fructan storage polysaccharide of roots and tubers of Dahlia and related plants. Inulin is not metabolised in human body and is readily filtered through the kidneys. It is, therefore, used in testing of kidney function, especially glomelular filtration.

B. Structural polysaccharide:

They are polysaccharides that take part in forming the structural frame work of the cell walls in plants and skeleton of animals. Structural polysaccharides are of two main types; chitin and cellulose.

1. Chitin:

It is a complex carbohydrate of heteropolysaccharide type which is found as the structural component of fungal walls (fungal cellulose) and exoskeleton of insects and crustacea. Chitin is a homopolymer of N-acetyl-D-glucosamine in β (1 → 4) linkage with an unbranched configu­ration.

2. Cellulose:

The most abundant cell-wall and structural polysaccharide in the plant world is cellulose, a linear polymer of D-glucose in β(1→4) linkage. Cellulose is, also found in some lower invertebrates. It is almost entirely of extracellular occurrence. On complete hydrolysis with strong acids, cellulose yields only D-glucose, but partial hydrolysis yields the reducing disaccharide cello- biose in which the linkage between the D-glucose units is β (1→4).

Cellulose is not attached by either α or β- amylase. Enzymes capable of hydrolyzing the β (1→4) linkages of cellulose are not secreted in the digestive tract of most mammals; and therefore they cannot use cellulose for food. However, the ruminants, e.g., the cow can utilize cellulose as food since bacteria in the rumen form the enzyme cellulose which hydrolyzes cellulose to D-glucose.

Cellulose molecules do not occur singly instead a number of chains are arranged in close anti- parallel fashion. The molecules are held together by intermolecular hydrogen bonds between hydroxyl group at position 6 of glucose residues of one molecule and glycosidic oxygen between two glucose residues of the adjacent molecule. There is also intramolecular strengthening of the chain by the formation of hydrogen bonds between hydroxyl group at position three and oxygen atom of the next residue.

C. Mucopolysaccharides:

Mucopolysaccharides or mucilages are quite common in both plants and animals. The most abundant acid mucopolysaccharide is hyaluronic acid present in cell coats and in the extracellular ground substance of the connective tissues of vertebrates.

Biological Significance of Carbohydrates:

1. Major source of energy:

Carbohydrates are essential to animals as they use them as respira­tory fuels. In animal cells, carbohydrates in the form of glucose and glycogen serve as an important source of energy for vital activities.

2. Structural components of cells:

Carbohydrates serve as an important structural material in some animals and in plants, where they constitute the cellulose framework.

3. Key role in metabolism:

Carbohydrates play a key role in the metabolism of amino acids and fatty acids.

4. Special functions:

Some carbohydrates have highly specific function, e.g., ribose in the nucleoproteins of the cells and galactose in certain lipids.